JP4326706B2 - Circuit board evaluation method, circuit board and manufacturing method thereof - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a circuit board used as a module on which an electronic component such as a semiconductor element is mounted.
[0002]
[Prior art]
Modules equipped with electronic components such as semiconductor elements are becoming more powerful with the recent development of electronics technology. This is to reduce solder voids that hinder heat conduction from the electronic component to the circuit board in order to efficiently and quickly release the generated heat to the outside of the system.
[0003]
The basic structure of the electronic component mounting circuit board is such that a metal circuit is formed on the surface of the ceramic substrate, a metal heat sink is formed on the back surface, and Ni plating is applied to the metal circuit and the metal heat sink. When the module is assembled, a semiconductor element is mounted on the metal circuit, and the metal heat sink surface is soldered to the base plate and fixed.
[0004]
As the material of the ceramic substrate, alumina, aluminum nitride, silicon nitride, etc., and as the material of the metal circuit, the metal heat sink, and the base plate, copper, aluminum, alloys thereof and the like are used. Also, the active metal brazing method using a brazing material containing an active metal component such as Ti, Zr, Hf and the like is mainly used for joining the ceramic substrate to the metal circuit and the metal heat sink. ing.
[0005]
When a thermal load such as a heat cycle is applied to the circuit board, thermal stress is generated due to the difference in thermal expansion between the ceramic substrate and the metal, and the ceramic substrate, the metal circuit, and the metal heat sink (hereinafter both referred to as “metal circuit etc.”). )) Cracks are generated in the ceramic substrate. This crack progresses with an increase in the number of cycles of the heat load, and in the extreme case, leads to dielectric breakdown. In order to suppress the occurrence of such cracks, Al having a low thermal stress has been used as a material for metal circuits.
[0006]
Since the Al circuit and the semiconductor element (silicon chip or the like) or the Al heat sink and the base plate are joined using Pb—Sn solder, it is necessary to perform Ni plating on the Al surface. Even when the metal circuit or the like is made of a Cu material, Ni plating is generally applied in order to prevent oxidation and deterioration of reliability due to reaction with solder. For soldering, there are a method of reflowing in the air or nitrogen using a flux, and a method of reflowing in a hydrogen atmosphere without using a flux. The latter is desirable for simplification of the process and environmental problems, but the reactivity between the plated Ni and the Sn of the solder is not good, so there is a problem that voids called solder voids are generated and the thermal resistance increases. It becomes.
[0007]
In order to solve this, a high-purity electric Ni plating method may be employed. However, since handling becomes complicated, not only does the cost increase, but there is a problem that it cannot be applied to a fine pattern.
[0008]
[Problems to be solved by the invention]
An object of the present invention is to provide a circuit board in which the solder void ratio between the electronic component and the circuit board is reduced by using inexpensive electroless Ni plating.
[0009]
[Means for Solving the Problems]
That is, the present invention is as follows.
(Claim 1) A circuit board evaluation method in which a metal circuit is formed on the front surface of a ceramic substrate and a metal heat sink is formed on the back surface, wherein Pb (90%)-Sn (10%) is formed on the surface of the metal circuit. A silicon chip is placed with a solder piece in between, and the temperature is increased at a rate of 15 to 20 ° C./min up to a temperature of 150 ° C. in a hydrogen atmosphere, and then at a rate of 2.3 to 2.5 ° C./min. After the temperature is raised to 350 ° C. ± 5 ° C., the module is assembled by using the circuit board by quickly cooling and soldering at room temperature and measuring the solder void ratio. A circuit board evaluation method characterized by knowing characteristics.
(Claim 2) A circuit board in which a metal circuit is formed on the surface of a ceramic substrate, a metal heat sink is formed on the back surface, and Ni plating is applied to the metal circuit and the metal heat sink, and is measured by the method of claim 1. A circuit board having a solder void ratio of less than 1.2%.
(Claim 3) After forming a metal circuit on the surface of the ceramic substrate and a metal heat sink on the back surface, electroless Ni plating is applied to the metal circuit and the metal heat sink, and then a temperature of 280 ± 10 ° C. in a vacuum of 1 Pa or less. The method for manufacturing a circuit board according to claim 2, wherein the heat treatment is performed for 10 to 30 minutes.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in more detail.
[0011]
The material of the ceramic substrate used in the present invention is preferably aluminum nitride or silicon nitride from the viewpoint of high reliability and high insulation. The thickness of the ceramic substrate can be freely changed according to the purpose. Usually, it is 0.635 mm, but a thin object of about 0.5 to 0.3 mm may be used. When it is desired to remarkably increase the withstand voltage under high voltage, a thickness of 1 to 3 mm is used.
[0012]
The material for the metal circuit or the like is preferably Al, Cu, or an Al—Cu alloy. These are used in the form of a single body or a laminated body such as a clad including this as a single layer. Al has a lower yield stress than Cu, is rich in plastic deformation, and can significantly reduce the thermal stress applied to the ceramic substrate when subjected to thermal stress such as a heat cycle, thus suppressing cracks generated in the ceramic substrate more than Cu. And a highly reliable circuit board.
[0013]
The thickness of the metal circuit is preferably 0.4 to 0.5 mm in the case of an Al circuit and 0.3 to 0.5 mm in the case of a Cu circuit in terms of electrical and thermal characteristics. On the other hand, the thickness of the metal heat radiating plate is determined so as not to cause warpage during soldering. Specifically, it is preferable that the Al heat sink is 0.1 to 0.4 mm, and the Cu heat sink is 0.15 to 0.4 mm.
[0014]
A metal circuit or the like is formed on a ceramic substrate by bonding a metal plate and a ceramic substrate and then etching, or a method of bonding a circuit punched from a metal plate and a pattern of a heat sink to the ceramic substrate. be able to.
[0015]
The surface of the metal circuit or the like before being subjected to Ni plating is preferably smoothed by grinding, physical polishing, chemical polishing or the like, and the surface roughness is preferably Ra ≦ 0.2 μm.
[0016]
The Ni plating is preferably an electroless method, which makes it possible to handle fine patterns. The Ni plating film thickness is preferably 2 to 8 μm.
[0017]
The circuit board of the present invention is a circuit board on which the above-described Ni plating is applied, and has a solder void ratio described below of less than 1.2%. The heat dissipation characteristics of a module assembled using such a circuit board are good even though the Ni plating method is an electroless method.
[0018]
The circuit board of the present invention can be produced by heat-treating the Ni-plated circuit board in a vacuum of 1 Pa or less at a temperature of 280 ± 10 ° C. for 10 to 30 minutes.
[0019]
The heat treatment of Ni plating in the present invention is performed in order to increase the crystallinity of Ni and increase the reactivity with the Sn component of the solder. By performing the heat treatment in a vacuum of 1 Pa or less, the oxide film on the plating surface is reduced and the reactivity with the Sn component is improved. When the degree of vacuum exceeds 1 Pa, oxidation of the Ni plating surface becomes remarkable, and the reactivity with the Sn component deteriorates conversely. On the other hand, if the heat treatment temperature is less than 270 ° C., the high crystallinity of Ni is not sufficient, and the reactivity with the Sn component cannot be increased to the target level. On the other hand, if it exceeds 290 ° C., the Ni plating film is hardened and the circuit board is damaged.
[0020]
Next, a method for evaluating a circuit board according to the present invention will be described. The evaluation method according to the present invention measures a solder void ratio after soldering a silicon chip to a metal circuit under a specific condition, and determines the circuit according to the size. This is to determine the heat dissipation characteristics of the module in which the substrate is used.
[0021]
In the present invention, a silicon chip is placed with a Pb (90%)-Sn (10%) solder piece sandwiched between metal circuits. As for the size of the solder pieces and the silicon chip, a plate having a bottom area of 5 to 25 mm ² × thickness of 0.5 to 1.0 mm is desirable.
[0022]
Soldering is performed at a temperature of 15 to 20 ° C./min up to a temperature of 150 ° C. in a hydrogen atmosphere, and then raised to a temperature of 350 ° C. ± 5 ° C. at a rate of 2.3 to 2.5 ° C./min. After that, it is carried out by natural cooling immediately at room temperature.
[0023]
Regarding the reason for raising the temperature up to 150 ° C. at 15 to 20 ° C./min, if it is slower than 15 ° C./min, the Ni plating surface is oxidized, and the original solder wettability cannot be correctly evaluated. Moreover, the apparatus becomes a large scale in order to make it faster than 20 ° C./min. Regarding the reason for raising the temperature up to 350 ° C. at 2.3 to 2.5 ° C./min, if it is slower than 2.3 ° C./min, the Ni plating surface is oxidized, and the original solder wettability is correctly evaluated. Can not do it. If it is faster than 2.5 ° C./min, the solder is not sufficiently melted and the original solder wettability cannot be correctly evaluated.
[0024]
The solder void ratio can be automatically measured using a soft X-ray flaw detector or an ultrasonic flaw detector. For example, a soft X-ray flaw detector is “PRO-TEST 100” manufactured by Softex Corporation, and an ultrasonic flaw detector is “HA-701” manufactured by Honda Electronics Co., Ltd. .
[0025]
When the solder void ratio is 2%, the heat dissipation characteristic of the module assembled using the circuit board changes greatly. When applied to mass production, it is desirable to manage so that the average value of 4 samples + 4σ is 2% or less.
[0026]
【Example】
Hereinafter, the present invention will be described more specifically with reference to examples and comparative examples.
[0027]
Examples 1-4 Comparative Examples 1-4 Reference Examples 1-4
An Al circuit forming Al plate (thickness 0.4 mm, purity> 99.9%) on the surface of a 0.635 mm × 58 × 33 mm aluminum nitride substrate (thermal conductivity 170 W / mK, three-point bending strength 400 MPa) On the back side, an Al heat sink forming Al plate (thickness 0.1 mm, purity> 99.9%) is placed in a hot press apparatus with a brazing material (Al—Cu (4%) alloy foil, thickness 30 μm) sandwiched between them. The temperature was 630 ° C., and the pressure was increased to 3 MPa to join.
[0028]
An etching resist was applied to the obtained joined body, and etching was performed with an FeCl ₃ solution, so that a circuit board having a solid Al circuit and a solid Al heat dissipation plate whose edges were just edged was manufactured.
[0029]
The obtained circuit board was subjected to electroless Ni—P plating (“Nimden SX” manufactured by Okuno Pharmaceutical Co., Ltd.) under various conditions shown in Table 1, followed by heat treatment, and the solder void ratio was measured as follows. Moreover, the heat dissipation characteristic of the module assembled using the said circuit board was measured according to the following. The results are shown in Table 1.
[0030]
(1) Measurement of Solder Void Ratio A silicon chip (bottom area 169 mm) with a Pb (90%)-Sn (10%) solder piece (bottom area 169 mm ² × thickness 0.1 mm plate) sandwiched between metal circuits of a circuit board ² × plate having a thickness of 0.4 mm). This was heated and soldered in various conditions shown in Table 1 in a hydrogen atmosphere, and the solder void ratio was measured using a soft X-ray flaw detector (“PRO-TEST 100” manufactured by Softex).
(2) Heat dissipation characteristics of the module Assembled into the simple module shown in Fig. 1, measured the thermal resistance between the silicon chip and the Al heat sink under the condition of 145W power supply to the silicon chip and Al heat sink temperature of 65 ° C, heat dissipation characteristics Was evaluated.
[0031]
[Table 1]
[0032]
From the comparison between Examples 1 to 3 and Reference Examples 1 to 4 in Table 1, it can be seen that the evaluation method of the present invention is highly reliable. Moreover, the module assembled using the circuit board obtained by the manufacturing method of this invention from the contrast with Examples 1-4 and Reference Examples 1-4 and Comparative Examples 1-4 has a small thermal resistance. I know that there is.
[0033]
【The invention's effect】
ADVANTAGE OF THE INVENTION According to this invention, the evaluation method of the circuit board which can know the thermal radiation characteristic when a module is assembled is provided.
[0034]
ADVANTAGE OF THE INVENTION According to this invention, the circuit board which can assemble the module excellent in the thermal radiation characteristic is provided.
[0035]
ADVANTAGE OF THE INVENTION According to this invention, the manufacturing method of the circuit board which can assemble the module excellent in the heat dissipation characteristic by the electroless Ni plating method is provided instead of the high-purity electric Ni plating method.
[Brief description of the drawings]
FIG. 1 is a simplified module assembly diagram for measuring heat dissipation characteristics.

Claims (3)

1.  A method for evaluating a circuit board in which a metal circuit is formed on the surface of a ceramic substrate and a metal heat sink is formed on the back surface, and a Pb (90%)-Sn (10%) solder piece is sandwiched between the surfaces of the metal circuit. A silicon chip is placed, and the temperature is raised at a rate of 15 to 20 ° C./min up to a temperature of 150 ° C. in a hydrogen atmosphere. After raising the temperature to ± 5 ° C, quickly cool it at room temperature and solder it, and measure the solder void ratio to know the heat dissipation characteristics of the module assembled using the circuit board. A method for evaluating a characteristic circuit board.
2. A circuit board comprising a ceramic circuit board having a metal circuit formed on the front surface and a metal heat sink formed on the back surface, the electroless Ni plating being applied to the metal circuit and the metal heat sink, and the solder measured by the method of claim 1 A circuit board having a void ratio smaller than 1.2%.
3.  After forming a metal circuit on the surface of the ceramic substrate and a metal heat sink on the back surface, electroless Ni plating is applied to the metal circuit and the metal heat sink, and then at a temperature of 280 ± 10 ° C. for 10 to 30 minutes in a vacuum of 1 Pa or less. The method of manufacturing a circuit board according to claim 2, wherein heat treatment is performed.

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